1. Field of the Invention
The present invention relates to a fuel injection method and device for engines which receives fuel from a common rail and injects the fuel from injectors into combustion chambers.
2. Description of the Prior Art
As for the fuel injection control in engines such as diesel engines, a common-rail type fuel injection system has been known which provides a high injection pressure and performs optimum control on the injection characteristics, such as fuel injection timing and the amount of fuel injected, according to the operating condition of the engine. The common-rail type fuel injection system stores in the common rail a fuel pressurized to a predetermined pressure by a fuel pump and then injects the stored fuel from injectors into corresponding combustion chambers. A controller controls the fuel pressure in the common rail and the operation of control valves for the injectors so that the pressurized fuel will be injected from each injector under optimum injection conditions according to the engine operating conditions.
The conventional common-rail type fuel injection system is described by referring to FIG. 7. The fuel is supplied to individual injectors 1 from a common rail 2 through branch pipes 3 that form a part of the fuel passage. The fuel, which was pumped by a feed pump 6 from a fuel tank 4 through a filter 5 and pressurized to a predetermined intake pressure, is delivered to a fuel pump 8 through a fuel pipe 7. The fuel pump 8 is a plunger type which is driven by the engine to raise the fuel pressure to a high pressure determined by the operating condition of the engine and deliver the pressurized fuel through a fuel pipe 9 to the common rail 2. The fuel is then stored temporarily in the common rail 2 at the elevated pressure, from which it is supplied to individual injectors 1. As many injectors 1 as the number of cylinders are provided in the engine. These injectors 1 are controlled by a controller 12, an electronic control unit, to inject fuel supplied from the common rail 2 into the corresponding combustion chambers at optimum timings and in optimum amounts. Because the pressure at which the fuel is injected from the injectors 1 is almost equal to the pressure of the fuel stored in the common rail 2, the injection pressure is controlled by controlling the fuel pressure in the common rail 2.
The fuel released from the fuel pump 8 is returned to the fuel tank 4 through a return pipe 10. Of the fuel supplied from the branch pipes 3 to the injectors 1, the fuel that was not used for injection into the combustion chambers is returned to the fuel tank 4 through a return pipe 11. The controller 12 is supplied with signals from a variety of sensors for detecting the engine operating condition, which include: engine cylinder determination and crank angle sensors for detecting an engine revolution speed Ne; an accelerator opening sensor for detecting the amount of accelerator operation Acc; a water temperature sensor for detecting the cooling water temperature; and an intake pipe inner pressure sensor for detecting the inner pressure of the intake pipe. The controller 12, based on these signals, controls the fuel injection characteristics of the injectors 1, i.e., the fuel injection timing and the amount of fuel to be injected so that the engine output will become optimum for the current engine operating condition. The common rail 2 is provided with a pressure sensor 13 which detects the fuel pressure in the common rail 2 and sends the detection signal to the controller 12. The controller 12 controls the amount of fuel delivered from the fuel pump 8 so that the fuel pressure in the common rail 2 is restored to a predetermined pressure after the common rail pressure has fallen as a result of fuel consumption from the common rail 2 for injection from the injectors 1.
The conventional common-rail type fuel injection system controls the fuel injection pressure to a target value according to the engine operating state and at the same time calculates injection characteristics corresponding to the operating state, i.e., the amount of fuel to be injected (determined by the fuel injection pressure and the fuel injection period) and the fuel injection timing, and controls the operation of the injectors according to the calculated results, thereby realizing the fuel injection characteristics conforming to the engine operating state. The common rail pressure that determines the injection pressure is raised by the fuel pump and controlled to a predetermined injection pressure by a pressure control valve (see Japanese Patent Publication No. 60020/1985 for example).
In the common-rail type fuel injection device, the controller sends a command pulse as an injection command signal to each of the solenoid valves of the injectors, and a needle valve is lifted by a solenoid that is energized in response to the command pulse to open nozzle holes provided at the nozzle end of each injector to inject fuel. It should be noted, however, that there is normally a time lag from the instant that the controller issues a command pulse to the solenoid valve to the instant that the fuel actually starts to be injected from the nozzle holes of the injector. This time lag results from a response delay present in a drive circuit, i.e., a delay after the command pulse has been issued from the controller to the solenoid until the solenoid is actually energized, and a mechanical delay after the solenoid has been energized to lift the needle valve until the fuel is injected from the nozzle holes. Further, even if the timings at which to turn on the command pulses from the controller are the same, the fuel injection start timings may differ from one injector to another because of variations among individual injectors and among cylinders (e.g., difference in distance between the pressure sensor and the injector).
The conventional fuel injection device for engines deals with such a time lag by taking it to be a fixed delay and not considering variations of the time lag. Hence, changes over time of the time lag and variations among individual injectors combine to prevent optimum combustion of fuel, causing deterioration of the exhaust emission performance and also engine vibrations due to differences in the combustion timing among cylinders.
The method and device for detecting fuel injection timings, disclosed in Japanese Patent Laid-Open No. 210174/1996 and intended to determine precise fuel injection timings in a diesel engine, performs a sequence of steps which involve monitoring a pressure in the fuel pipe connecting a fuel injection pump of the diesel engine and the fuel injection nozzles, detecting a first fall in the monitored pressure of a magnitude greater than a predetermined value after the monitored pressure has reached a predetermined high pressure value, and then determining a pressure fall start timing to be a fuel injection start timing. The above-described method and device for detecting the fuel injection timing, however, concerns a fuel injection system using a distribution or column type pump that distributes fuel to the injectors, and is not intended for application to the common-rail type fuel injection system. That is, the type of the system for which the above method is intended differs from the common-rail type fuel injection system under consideration in this invention.